Method of preparing highly refractory bodies



Patented Jan. 12, 1954 asst? METHOD OF PREPARING HIGHLY REFRACTORY BODIES No Drawing. AppIicatiOnMarch18,1950, Serial No. 150,543

Qla ms 1 This-invention relates to a: method: of prepare ing metali bodies resistant to oxidation at ele-.. vated'temperatures and more particularlyv to such bodies. formed of the refractory' metals, molybdenum, tungsten, tantalumand columbium, which. are'providedwith a coatingor skin so as to render the metal base resistant to. oxidation at elevated temperatures.

The refractorymetals, molybdenum, tungsten, tantalum and columbium, have highly desirable properties and characteristics for manypur-. poses; Molybdenum and tungsten, for example, retain their mechanical properties at elevated temperatures. Molybdenum, tungsten and tantalum are very desirable for use inelectrical furnace heating elements. However, in order to prevent oxidation of these metals it is necessary to exclude oxygen. In the use of molybdenum and tungsten elements for suchpurpose, a continuous flow of hydrogen is maintained over the heated resistance elements; Tantalum and columbium are highly resistant to chemical attack, particularly acids, however they must be handled cautiously at; higher temperatures to prevent the absorption of gases. Heating of tantalum and columbium inthe atmosphere or in gases. results in an embrittlement of the metal.

The principal. purpose of the present invention is, to provide a method for producingrefractory metal bodies. formed; of these metals having a coating or skin which is resistant to oxidation at elevated temperatures and which protects the base or core from oxidation at such temperatures.

Other objects and advantages of'thi's invention will become apparent from the description which follows.

The present invention contemplates a method for providing these refractory metals normally oxidizable at high temperatures with an integral coating or skin of the refractory metal-silicon alloy or intermetallic. com-pound.

, Methods are known whereby silicon may be deposited on refractory metals. The protection, afforded by the silicon coating is of course limited somewhat by the relatively low melting point of silicon and its tendency to alloy with the base material. Although silicon itself has excellent oxidation resistance and protective silicon coatings may be formed on these metals, it is impos sible to maintain the silicon coating, as such, over extended periods at high temperatures because the silicon is gradually consumed in the formation of alloys or intermetallic compounds with the metal base. In the course of this alloy-. ingrea'ction, the continuity of thecoating :is fre- 2 quently destroyed and the lifeofthe coated body, particularly at higher temperatures, is generally much shorter than the life of the bodies coated with a skin of the alloys or intermetallic come pounds as prepared bythe method of this in- Vention.

In contra distinction to these prior methods whereby the base material is coated with silicon, our method provides a surface layer orcoatingwhich consists of alloys or intermetalliccom pounds of the metal base and silicon rather than merely a coating of silicon. This alloyor intermetallic compound coating or skin is formed by depositing silicon on the heated metal from a vapor phase and under conditions whereby the compound or alloys are formed directly. The metal may be heated by any desired means and: is preferably raised to the deposition temperature before silicon containing vapors are brought into contact with the body.

We have discovered that the type of coating formed during the deposition operation dependent upon the temperature of the body during plating. If the temperature of the body, at the time of deposition of the silicon, is below the melting point of silicon, the coating may consist predominantly of silicon, dependent upon the precise temperature. When the temperature approaches the melting point of silicon, some of the deposited silicon may alloy or interactwit-h the metal, however, the protective qualities, are, not appreciably different from those 'of the sub: stantial-ly pure silicon coatings.

In accordance with our method, the metal body is maintained at a temperature at least as high as the melting point of silicon during the deposi 'tion of the silicon. The metal body ispreferably raised to the plating temperature and a mixture of hydrogen and the vaporsoi a silicon halide such as a silicon chloride, silicon bromide and the like, is then passed over the heated body. In some instances, it may be desirable to heat the body to a higher temperature before plating and reduce the temperature before the hydrogenvapor mixture is over th body. As th vapors are decomposed the silicon deposits or plat ut n th m tal cdync the t mn a= ture of the body is above, the melting point of silicon, the silicon alloys or reacts with the metal o f m a skin, or coatin The alloys or intormetallic compounds formed by maintaining the body at the elevated tem-J peratures during plating will vary in the molecu: lar ratio of silicon to. refractory metal of from about 1:1 to about 2:1. A plating temperature of the order of about 1600 C. appears to promote the formation of an alloy or intermetallic compound of a composition corresponding to a molecular ratio of silicon to the refractory metal of about 2:1 which exhibits a maximum protective quality. The thickness of the skin or coating may be controlled by regulating both the time and plating temperature. At lower temperatures the time must be extended to form a skin or coating of predetermined thickness and insure a substantially complete alloying of the silicon and metal. At higher temperatures there is almost an immediate alloying as the silicon deposits on the metal and the thickness of the alloy skin or coating may be controlled by regulating the plating period. At the conclusion of the desired plating period the flow of the hydrogen-vapor mixture is arrested and is purged from the plating chamber by passing hydrogen or an inert gas through the chamber. Care should be exercised in cooling the coated body so as to insure a substantially complete conversion of all of the silicon to the alloy or intermetallic com pound.

Our method may be illustrated specifically by reference to the coating of molybdenum wire or rod. The wire or rod is preferably heated in relatively pure hydrogen for several minutes at atemperature above the plating temperature. The heating may be accomplished by passing an electric current through the wire or rod, or it may be heated by induction. The temperature is then reduced to a temperature at least as high as the melting point of silicon, for example, 1500 C. to 1600 C., and a mixture of hydrogen and silicon tetrachloride vapors is allowed to pass over the heated wire or rod. The silicon tetrachloride is decomposed thereby releasing silicon which deposits or plates out on the wire or rod. Since the temperature of the wire or rod exceeds the melting point of silicon, there is a reaction between the surface portion of metal and the deposited silicon to form the intermetallic compounds or alloys. As described above, the plating temperature and time may be varied so as to control the rate of deposition and the alloying reaction. The thickness of the skin or integral coating may be controlled by a regulation of the time during which silicon tetrachloride and hydrogen are passed over the heated metal. At

the conclusion of the plating period the chamher is purged with hydrogen. For example, the following schedule has been applied for the coating of wire, rod and strips of molybdenum for a plating temperature of 1600 C., that is, the molybdenum is maintained at this temperature during the plating period:

Plating Time, seconds g fig f samples of untreated or unco'ated wires were heated to about 1000 C. in air by passing an electric current through the wires. The average life of these samples at this temperature was as follows:

The oxidation resistance of these metals at elevated temperatures may be further increased by applying the coating or skin in successive layers with intermediate soaking or heat treatment periods. During the intermediate soaking periods the temperature of the metal body is maintained sufiiciently high to insure a complete formation of the alloy or intermetallic compound. For example, a molybdenum wire was subjected to four plating or coating periods, each period being followed by a heat treatment consisting of maintaining the coated wire at a temperature of about 1750 C. in hydrogen for about five minutes so as to insure a complete conversion of the silicon to the alloy or intermetallic compound. This wire had a life at 1700 C. (3092 F.) in air of 159.6 hours.

It is to be understood that the foregoing description is merely illustrative of our invention and that the specific examples included above are not intended as limitations. Although the specific illustration describes the method as applied to molybdenum wire or rod, it is to be understood that the other metals, tungsten, tantalum and columbium, may be provided with the alloy or intermetallic compound coatings in a similar manner. The specific examples have described only the preparation of coated or alloyed wire. Bars, strips and other bodies have been similarly protected against oxidation at elevated temperatures. The description of the treatment of wire or rod and the testing of wire affords a simple and ready illustration of the effectiveness of the coatings or skins and of the comparison of the bodies treated in accordance with the method of this invention with the metals themselves.

We claim:

1. The method of forming refractory metal bodies resistant to oxidation at temperatures above about 1000" C., which comprises heating a refractory metal body to a temperature of at least the melting point of silicon, depositing silicon on the metal body by passing a mixture of hydrogen and vapors of a silicon halide over the metal body and reacting the deposited silicon with the surface portions of the metal body by maintaining the metal body at the elevated temperature of at least the melting point of silicon, to form a coating on the said body of an alloy or intermetallic compound of the refractory metal and silicon.

2. The method of forming refractory metal bodies resistant to oxidation at temperatures above about 1000 C, which comprises heating a refractory metal body to a temperature of at leastthe melting point of silicon, depositing silicon on the metal body by passing a mixture-of hydrogen and vapors of silicon tetrachloride over the metal body and reacting the deposited silicon with the surface portions bf the metal body by maintaining the metal body at the elevated temperature of at least the melting :point of 'sil-i con, to form a coating on the "said body of an alloy or intermetailiccompound 0f the refractory metal and silicon 3. The method of forming refractory metal bodies resistant to cbriciation at temperatures above about 1000 C., which comprises heating a molybdenum body to a temperature of at least the melting point of silicon, depositing silicon on the body by passin amixture or hydrogen and vapors of silicon tetrachloride over th'e'body and reacting the deposited silicon with the surface portions of the body by maintaining the body at the "elevated temperature of at least the melting po nt of Silicon, to fo'ffil a coating on the said body of an alloy or 'in'termetallic cb'zn'pdiih'd b'riiidlybdenumaha silicon. g

i. The method "of ire-rul ng refractory iiietal bodies resistant to shaman at temperatures above about 1000 0., which comprises heating a tungsten body to a temperature of at least the melting point of silicon, depositing silicon on the body by passing a mixture of hydrogen and vapors of silicon tetrachloride over the body and reacting the deposited silicon with the surface portions of the body by maintaining the body at the elevated temperature of at least the melting point of silicon, to form a coating on the said body of an alloy or intermetallic compound of tungsten and silicon. 5. The method of forming refractory metal bodies resistant to oxidation at temperatures above about 1000 C., which comprises heating a tantalum body to a temperature of at least the melting point of silicon, depositing silicon on the body by passing a mixture of hydrogen and vapors of silicon tetrachloride over the body and reacting the deposited silicon with the surface portions of the body by maintaining the body at the elevated temperature of at least the melting point of silicon, to form a coating on the said body of an alloy or intermetallic compound of tantalum and silicon.

6. The method of forming refractory metal bodies resistant to oxidation at temperatures above about 1000 0., which comprises heating a columbium body to a temperature of at least the melting point of silicon, depositing silicon on the body by passing a mixture of hydrogen and vapors of silicon tetrachloride over the body and reacting the deposited silicon with the surface portions of the body by maintaining the body at the elevated temperature of at least the melting point of silicon, to form a coating on the said body of an alloy or intermetallic compound of columbium and silicon.

7. The method of forming refractory metal bodies resistant to oxidation at temperatures above about 1000 0., which comprises heating a melybeeimm body to a tem erature of between the melting point of *silicon'and about 1600 0;, depositing silicon on theho'dy by passing a mixture'of. hydrogen and vapors of silicon tetrachloride over the bodyand reacting the deposited si-liccn with the surface portions of the body by maintaining the-'bo'dy atfithe elevated temperature'of 'a't' least the melting point of silicon, to

refractory metal body to a temperature of between the melting point of silicon and about 1600" 0., depositing silicon on the body by passing a mixture of hydrogen and vapors of silicon tetrachloride over the body and reacting the deposited silicon with the surface portions of the body by maintaining the body at the elevated temperature of at least the melting point of silicon, to form a coating on the said body of an alley or intermetallic compound of the refractory metal and silicon.

8. The method of forming refractory metal bodies resistant to oxidation at temperatures above about 1000 C., which comprises heating a form a. coating ran the said body of an alloy or intermetallichompouhd of molybdenum and silicoil.

9. The method of fomi'ing refractory metal bodies resistant to oxidation at temperatures above about 1000 'C-.-, which comprises heatingza tungsten bodyto a temperature of between the melting point "or silicon and about -1 600 C., "depositing silicon on the body by passing 'a mixture of hydrogen and vapors 'of silicon tetrachloride over the 'body and reacting "the deposited silicon with thesurface portions'of the body by maintain-- ing the *body at the elevated temperature or at least the melting .pointof silicon, to form a moating on the said body of an alloy or intermetallic compound of tungsten and silicon.

10. The method of forming refractory metal bodies resistant to oxidation at temperatures above about 1000 C., which comprises heating a tantalum body to a temperature of between the melting point of silicon and about 1600 C., depositing silicon on the body by passing a mixture of hydrogen and vapors of silicon tetrachloride over the body and reacting the deposited silicon with the surface portions of the body by maintaining the body at the elevated temperature of at least the melting point of silicon, to form a coating on the said body of an alloy or intermetallic compound of tantalum and silicon.

11. The method of forming refractory metal bodies resistant to oxidation at temperatures above about 1000 C. which comprises heating a columbium body to a temperature of between the melting point of silicon and about 1600 C., depositing silicon on the body by passing a mixture of hydrogen and vapors of silicon tetrachloride over the body reacting the deposited silicon with the surface portions of the body by maintaining the body at the elevated temperature of at least the melting point of silicon, to form a coating on the said body of an alloy or intermetallic compound of columbium and silicon.

12. The method of forming refractory metal bodies resistant to oxidation at temperatures above about 1000 0., which comprises heating a molybdenum body in a hydrogen atmosphere to a temperature exceeding about 1600 C., reducing the temperature of the body to between about 1500 C. and about 1600 C., and passing over said body a mixture of hydrogen and vapors of silicon tetrachloride while maintaining the same at the temperature between about 1500 C. and about l600 0., to form a coating on the said body consisting essentially of an alloy of molybdenum and silicon.

13. The method of forming refractory metal bodies resistant to oxidation at temperatures above about 1000 C., which comprises heating a refractory metal body in a hydrogen atmosphere to a temperature exceeding about 1600" C., reducing the temperature of the body to between about 1500 C. and about 1600 C., and passing over said body a mixture of hydrogen and vapors of a silicon halide while maintaining the same at the temperature between about 1500 C. and about 1600 C., to form a coating on the said body consistingessentially of an alloy of the said refractory metaland silicon.

- 14. The method of forming refractory metal bodies resistant to oxidation 7 at temperatures above about 1000 (3;, which comprises heating a molybdenum body to atemperature of at least the melting point of silicon, depositing silicon on the metal body by passing a mixture of hydrogen and vapors of a silicon halide over the metal body and reacting the deposited silicon with the surface portions of the metal body by maintaining the metal body at the elevated temperature of at least the melting point of silicon, to form a coating on the said body of an alloy or'intermetallic compound of molybdenum and silicon. r 15. The method of forming refractory metal bodies resistant to oxidation at temperatures above about 1000 C., which comprises heating a refractory metal body to a temperature of between the melting point of silicon and about 1600 0., depositing silicon on the body by passing a mixture of hydrogen and vapors of a silicon halide over the body and reacting the deposited silicon with the surface portions of the body by maintaining the body at the elevated temperature of at least the melting point of silicon, to form a coating on the said body of an alloy or intermetallic compound of the refractory metal and silicon.

IVOR E. CAMPBELL. BRUCE W. GONSER.

CARROLL F. POWELL.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,853,370 Marshall Apr. 12, 1932 1,964,322 Hyde June 26, 1934 2,157,902 Ihrig May 9, 1939 2,306,222 Pat-node Dec. 22, 1942 2,412,470 Norton Dec. 10, 1946 2,501,051 Henderson et a1. Mar. 21, 195i) 

1. THE METHOD OF FORMING REFRACTORY METAL BODIES RESISTANT TO OXIDATION AT TEMPERATURES ABOVE ABOUT 1000* C., WHICH COMPRISES HEATING A REFRACTORY METAL BODY TO A TEMPERATURE OF AT LEAST THE MELTING POINT OF SILICON, DEPOSITING SILICON ON THE METAL BODY BY PASSING A MIXTURE OF HYDROGEN AND VAPORS OF A SILCON HALIDE OVER THE METAL BODY AND REACTING THE DEPOSITED SILICON WITH THE SURFACE PORTIONS OF THE METAL BODY BY MAINTAINING THE METAL BODY AT THE ELEVATED TEMPERATURE OF ATLEAST THE MELTING POINT OF SILICON, TO FORM A COATING ON THE SAID BODY OF AN ALLOY OR INTERMETALLIC COMPOUND OF THE REFRACTORY METAL AND SILICON. 